Any CH wiring experts out there!

Current heating wiring was put in by someone who practiced creative wiring -
hence no wiring centre

System is semi gravity

There is a switched live on the programmer./thermo stat for CH ON or HW ON.
Output goes live when heat called for. There are output for live when off
(ie. spdt) but these are currently not connected.

When HW ON - there is power to Swl L in the boiler and the boiler fires up.

When CH ON - there is power to L in the boiler and the pump runs (no boiler)
and Swl L is off. Pump is connected to the boiler

When HW & CH is on, the pump runs and the boiler is fired. (i.e we get
central heating and HW)


We therefore have the following situation:

Both CH & HW On we get Hot Water and Heating - i.e OK
If HW Only is on: We get Hot Water - No Heating i.e OK
If only CH Only is on: We get Pump Only i.e
Problem


I cannot wire the CH On output to Swl as it would work backwards - i.e HW On
we would then get pump.

How can this be rewired to give CH Only. I reaslise that there is currently
no valve on the system, so HW temp control is only provided when in HW only
mode. If CH is running then there is no control of HW temp.

How should a valve be wired in to given addtional control?